Nonmyeloablative allogeneic hematopoietic cell transplantation (HCT) is a major advance in the curative treatment of patients with hematologic malignancies, with significantly decreased regimen-related toxicity. However, disease relapse remains a substantial problem. Nonmyeloablative allogeneic HCT relies on the T cell mediated graft-versus-host (GVH) reaction for elimination of the underlying hematologic disease. Strategies to enhance the control of the GVH reaction and the graft-versus-leukemia (GVL) effect without GVH disease (GVHD) are needed to prevent or treat disease relapse following allogeneic HCT. Using the dog model of mixed hematopoietic chimerism established with a nonmyeloablative regimen, we propose to enhance the GVH-effect with gene modified donor lymphocyte infusion (DLI). Long-term, stable mixed hematopoietic chimerism has been reliably established following 2 Gy total body irradiation, transplantation of DLA-identical littermate marrow and postgrafting immuncsuppression with combined mycophenolate mofetil (MMF) and cyclosporine for 4 and 5 weeks after transplant, respectively. Since complete (100%) donor chimerism is necessary to treat patients with hematologic malignancies, we will convert mixed to complete donor chimerism in this dog model with donor T cells that have an immunologic advantage over the host. Donor T-cells will be transduced with a lentiviral vector containing a mutant inosine monophosphate dehydrogenase II (IMPDH*) gene that renders cells resistant to MMF. We will ask if IMPDH* transduced donor T-cells infused into stable mixed chimeric dogs treated concurrently with MMF will convert mixed to complete donor chimerism. We hypothesize that MMF will selectively suppress the host and prevent an immune response to the IMPDH* transgene while permitting the GVH-effect of IMPDH* transduced donor T-cells. If dogs develop GVHD, future studies will aim to ablate donor T-cells transduced with IMPDH* and the herpes simplex thymidine kinase gene with the antiviral drug ganciclovir. If successful, this study could translate into improved control of the GVH reaction and improved survival after allogeneic HCT.